The present invention relates to a resistance adjusting type heater unit which can be suitably used for purification of automobile exhaust gas, etc.
Porous ceramic honeycomb structures have been known to be used as, for example, a catalyst or a catalyst carrier for conversion of nitrogen oxides (NO.sub.x), carbon monoxide (CO) and hydrocarbons (HC) present in the exhaust gas emitted from the internal combustion engines of automobiles, etc. Additionally, metallic honeycomb structures have come to draw attention recently.
Meanwhile, with the stricter regulation for exhaust gas, it is desired to develop a heater or the like capable of reducing emission at cold start.
As a honeycomb structure as mentioned above, there is known, for example, a technique described in Japanese Utility Model Laid-Open No. 67609/1988. In this document there is disclosed a catalytic converter comprising (a) a ceramic main monolith catalyst and (b) an electrically heatable metal monolith catalyst provided upstream of the ceramic main monolith catalyst (a) in close vicinity thereto, consisting of a metal carrier and alumina coated thereon.
In the catalytic converter described in the document, the metal monolith catalyst as a preheater provided upstream of the ceramic main monolith catalyst in close vicinity thereto is merely a foil type metal honeycomb structure wherein electricity is passed from the inner periphery to the outer periphery for heating the structure and which has no resistance adjusting means (that is, only the material, dimension and rib thickness are specified and no resistance control is made). Therefore, there was a problem of insufficient heat-generating property.
Hence, the present applicant proposed in U.S. Pat. No. 5,063,029 a heater comprising a honeycomb structure, at least two electrodes for enabling electrical heating of the honeycomb structure, provided on the honeycomb structure, and a resistance adjusting means provided between the electrodes. In this heater, a heat-resistant inorganic adhesive of zirconia type is filled at the outer periphery of each slit to isolate the slits from each other.
This heater is capable of controlling its heat-generating property and is useful for reduction in automobile emission at cold start; however, it had a problem that the inorganic adhesive might be detached under the severe driving conditions of automobiles, particularly vibration and thermal shock.